The object of devices for artificially weathering of material specimens is to estimate the useful life of materials which when put to use are permanently exposed to natural weather conditions and thus deteriorate under such climatic influencing effects as the light and heat of the sun, moisture and the like. For a good simulation of natural weather conditions the spectral energy distribution of the light generated in the device needs to correspond as best possible to that of natural sunlight, this being the reason why xenon radiators are employed as a rule in such devices as the sources of radiation. Accelerated aging-testing the materials is substantially achieved by irradiating the specimens much more intensively than under natural conditions, resulting in aging of the specimens being accelerated so that an indication as to the long-term aging response of a material specimen is obtained after a relatively short period of time.
Most of the specimens tested in artificial weathering apparatuses are made of polymer materials, the deterioration of which due to weathering is substantially caused by the UV component of solar radiation. The photochemical primary processes involved, in other words the absorption of photons and the generation of stimulated conditions or free radicals, are not a function of the temperature in a first approximation, whereas the subsequent steps in the reaction with the polymers or additives may depend on the temperature, so that the observed aging of the materials is likewise a function of the temperature. To what degree this temperature dependency holds depends on the material and the observed change in properties involved.
To take this into account, it is generally so that in artificially weathering of polymer materials room temperature and/or the specimen temperature is maintained constant. Maintaining constant and knowledge of the temperatures concerned is necessary because of aging being a function of temperature to permit comparing the results of the various weathering sequences to each other.
Since directly measuring the temperature of the material specimens being tested is problematic, temperature sensors are employed in weathering apparatuses, whose sensed temperature is a measure of the specimen temperature. For this purpose use is made, as a rule, of planar temperature sensors, particularly so-called black standard sensors, black panel sensors and white standard sensors. These are usually structured so that they comprise a metal plate having a lacquered surface facing the radiation source of the weathering apparatus during operation and a thermistor such as a platinum resistor on the reverse side of the metal plate. Within the weathering apparatus the temperature sensors are subjected to precisely the same conditions as the material specimens, i.e. they being particularly exposed to the radiation field of the radiation source and the remaining conditions set within the weathering chamber.
A distinction is made between so-called dynamic weathering apparatuses, in which the material specimens and the temperature sensors on a specimen mount are caused to rotate about a radiation source, and static weathering apparatuses in which the material specimens and the temperature sensors are maintained stationary.
In static weathering apparatuses such as, for example, the “Suntest” apparatus of the present assignee the material specimens and the described planar temperature sensors are mounted directly juxtaposed on a base plate of the apparatus within a weathering chamber of the weathering apparatus and exposed to the various weathering sequences of the weathering apparatus. These weathering sequences mostly involve, among other things, a rain phase in which the material specimens are sprayed with ultrapure water under controlled conditions. After the rain phase in which the temperature sensors are wetted with water these are required to resume their normal function as quickly as possible. This makes it necessary that the temperature sensors are relieved of water droplets as quickly and as thoroughly as possible. For this purpose the temperature sensors are arranged slightly inclined to the base plate of the weathering apparatus so that the water droplets can run off by the force of gravity. It has, however, been discovered that although the water droplets run to the bottom edge of the temperature sensor they fail to run across the edge and thus collect at the edge. When this happens, however, the temperature sensors are unable to fully function and it takes quite a long time before, for example, a black standard sensor has retained its wanted value in being fully functionable. If nothing is done, this delay lasts as long as the water droplets having collected at the edge need to evaporate. This, however, drags out the weathering sequences intolerably.